Ukraine and its backers in Europe insist that the United States and Russia cannot decide on land swaps behind their backs at a summit this week, but the Europeans concede that Moscow is unlikely to give up control of Ukrainian land it holds.

Ukrainian forces are increasing the intensity of long-range drone strikes deep into Russia ahead of Friday's planned meeting between Presidents Trump and Putin.

"The presidents will undoubtedly focus on discussing ways to reach a sustainable settlement to the Ukraine crisis," a Kremlin aide said on Saturday.